Smoking marijuana doesn't boost your chances of getting lung cancer, even if you're a long-time, heavy dope user, according to a new study.

The U.S. researchers were surprised by their findings, presented this week at a conference of the American Thoracic Society in San Diego. They had expected the controversial weed would jack up cancer risk, just like smoking tobacco.

In fact, previous studies have shown that marijuana tar contains 50 per cent higher concentrations of chemicals linked to lung cancer, compared with tobacco, said lead researcher Donald Tashkin of the University of California at Los Angeles. What's more, marijuana smokers hold their breath about four times longer than tobacco consumers, allowing more time for the hazardous particles to deposit in the lungs.

Even so, the study of more than 2,000 people with different smoking habits found no link between dope smoking and lung, head or neck cancers.

Dr. Tashkin speculates that THC, a chemical in marijuana smoke, "may encourage aging cells to die earlier and therefore be less likely to undergo cancerous transformation."

Despite the reassuring findings, Dr. Tashkin isn't encouraging people to light up a joint. "I wouldn't give any smoke substance a clean bill of health," he told Bloomberg News. There is still reason to believe dope might contribute to other lung ailments such as bronchitis and respiratory infections.

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